currently there's a limitation with logging using variadic templates:

eg:
the following is not allowed because it's ambiguous:
void DEBUG(T...)(T a, string file=__FILE__) {...}

proposal:
create a new kind of optional argument that's uninstantiable by user code, only by the compiler. We could use the following new syntax "==" instead of "=":
void DEBUG(T...)(T a, string file==__FILE__) {write(file,":"); foreach(ai ;a) write(ai," "); }

user code:
DEBUG(1+1, 2.2); //at file=__FILE__="myfun.d"
//this will print:"myfun.d:1+1 2.2"

the compiler would apply the same rules as for deducing the variadic T in "void DEBUG(T...)(T a)" and append one more arg to it (file). Note that file can't directly be instantiated by user code, it has to be implicit, to avoid the ambiguity.

This would be also very helpful in conjunction with my previous proposal earlier today ("feature request: __ARGS__ for logging (cf __FILE__, __LINE__, __FUNC___)". One could simply write:
---
void DEBUG(T...)(T a, string[]args==__ARGS__){
foreach(ai,vai;lockstep(a,args) writeln(vai,"=",ai,"; ");
}
int a=3,b=4;
DEBUG(1+2,3*3,a,b);//prints 1+2=3; 3*3=9;a=3;b=4;
---
Plain dead simple logging that just works with any number of args and avoids verbosity of tuple syntax; very useful for quick debugging.

Note: we can have more such special optional args at the end:
void DEBUG(T...)(T a, string[]args==__ARGS__, string file==__FILE__)
the limitation: they have to be implicitly convertible as above by such macros.